I think perhaps a more interesting way of looking at this is to simply take a category, plot all of the posts over the pseudo-time x-axis, then calculate a leas

I think perhaps a more interesting way of looking at this is to simply take a category, plot all of the posts over the pseudo-time x-axis, then calculate a least squares trend line for that category. Categories that slope down on shrinking, and categories that slope up are growing. In the end, a graph isn't even necessary, just a list of categories ordered by slope.